Method and compositions for making refractory shapes having dense, carbon free surfaces and shapes made therefrom

ABSTRACT

A dense carbon free surface is provided on a carbon bonded refractory shape which is particularly useful as a pouring tube liner to prevent the formation and build-up of alumina particles which may cause blockage of the tube. The carbon free, dense layer is also hard and erosion resistant making it ideal for use in other applications such as stopper rod noses. The material comprises a refractory mix having a major component of refractory oxides such as alumina and zirconia-mullite. Less than 10 wt. % carbon in the form of graphite and binder is in the mix plus about 2-5 wt. % of a metal such as silicon and an effective mount of sintering aids is also present. The pressed shape is preheated to a temperature of 1000° -1400° C. in air to oxidize the exposed surface of the shape and then to sinter the oxidized surface to form the desired dense, carbon free surface over the entire shape or on selected surface portions thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 08/490,792,filed Jun. 15, 1995.

BACKGROUND OF THE INVENTION

The present invention relates generally to refractory materials and,more particularly, to refractory compositions and shapes made therefromfor use in steelmaking operations. It is common practice in thecontinuous casting of steel to utilize special refractory materials andshapes to control the flow of the molten steel and to protect the liquidmetal against oxidation as it is teemed from a ladle to a tundish andthence to the mold or molds. Among such special refractory shapes areslidegate plates and/or stopper rods which are used to control the flowof the molten metal. In addition, specially shaped refractory pouringtubes are associated with the ladle and the tundish, such as submergedladle shrouds and nozzles which are employed to protect the molten metalfrom ambient oxidation during the teeming/casting operations. Theserefractory shapes are subjected to severe operating conditions and mustbe able to withstand thermal shock, as well as the chemical/erosiveattack of molten steel and slag.

The above-mentioned steelmaking refractories are commonly made fromcarbon containing compositions, including one or more refractory grainssuch as alumina, zirconia, clays, magnesia, silicon carbide, silica orother dense grains of specific mesh size. These refractories alsogenerally contain significant amounts of carbon in the form of graphite,carbon black, coke and like carbon sources plus a carbonaceous binderderived from sources such as pitch or resin. Such pressed and firedrefractory shapes are known to possess good physical properties,particularly thermal shock, making them suitable for use in this severeoperating environment.

It is well-known that during steelmaking operations, considerableamounts of oxygen may dissolve in the molten metal. In order toeliminate unwanted porosity, cracks and other internal defects whichwould lower the quality of the finished steel resulting from thepresence of oxygen, molten steels are commonly de-oxidized or "killed"by the addition of aluminum metal, ferromanganese or ferrosilicon. Inthe common case of aluminum killed steel, the aluminum addition reactswith dissolved oxygen or iron oxide to form finely dispersed aluminumoxide in the melt, some of which remains as highly dispersedmicroparticles in the solidified steel while a portion floats into theslag above the molten steel. During continuous casting, this extremelyfine dispersed portion of alumina has a tendency to either precipitateout of the molten steel on to the cooler refractory surfaces or itreacts and deposits on the ceramic refractory surfaces in contacttherewith. This gradual build-up of alumina causes problems in thecontrol of the now of molten steel and may eventually cause blockage inthe pouring nozzles. The precipitated alumina in the melt has aparticular affinity to the conventional carbon bonded, alumina-graphitematerial, commonly used in the manufacture of ladle shrouds andsubmerged pouring nozzles. The alumina will continue to build up in thebore of the nozzle until the flow of molten steel is reduced to a pointwhere the pouring tube must be lanced open by an oxygen torch, or therefractory tube is discarded. If oxygen lancing becomes necessary, thecasting process is disrupted, costing time and money, because castingefficiency decreases and the quality of the steel must be downgraded dueto the disruption in the continuous casting rim. Needless to say, thealumina blockage problem in continuous casting tubes decreases theexpected useful life of the refractories and is very costly to steelproducers. In aluminum killed steels where high dissolved oxygenconcentrations are expected, the useful life of a submerged pouringnozzle may be limited to 2-3 ladles due to heavy alumina build-up on theinterior diameter of the casting tubes.

Heretofore, one of the solutions to this alumina build-up problem hasbeen the development of an argon injected nozzle which allows highpressure argon to permeate the porous interior diameter of the nozzleduring casting. In this approach, it is believed that a protective layerof inert gas hinders the bonding of the dispersed alumina to therefractory. The argon also reduces the oxygen partial pressure at therefractory-molten metal interface, again decreasing the possibility foradherence of alumina deposits. Such a gas permeable immersion pouringnozzle is disclosed in U.K. Patent Application GB 2,111,880A to Gruneret al. Naturally, the argon injection approach extends the nozzle lifebut at increased cost due to the added expense of utilizing largevolumes of argon during casting as well as the increased manufacturingcosts of the more complex argon injection nozzle.

It has also been proposed to provide a pouring nozzle with a lowermelting point nozzle liner composition to prevent alumina build-uptherein. Liner materials include the use of calcia, magnesia andalumina, as disclosed in U.K. Patent Application GB 2,170,131A to Tate.These materials develop low melting eutectics causing the liner to washout of the nozzle as alumina is deposited. The melting action of theliner prevents the alumina build-up within the bore and allows for thefree flow of molten steel therethrough. Also reported to be effective inprevention of alumina adhesion is a sleeve of magnesia disclosed in U.K.Patent Application GB 2,135,918 to Rosenstock et al.

In U.S. Pat. No. 4,870,037 to Hoggard et al., owned by the assignee ofthe instant invention, an anti-build-up liner of carbon bonded,sialon-graphite refractory composition is disclosed. A still furtherattempt to minimize alumina build-up in pouring nozzles is set forth incommonly owned U.S. Pat. No. 4,913,408 to Hoggard et al., whichdiscloses a nozzle liner composition of carbon and a composite selectedfrom the group consisting of zirconia and O'-Sialon and zirconia insilicon oxynitride compositions. While these compositions have somewhatimproved the anti-build-up properties of the refractory shapes, it isnoted that the sialon constituent is a relatively expensive refractorymaterial which necessarily increases the cost of the nozzle.

More recently, in commonly owned U.S. Pat. No. 5,370,370 to Benson, acarbon bonded refractory body is disclosed for use in casting aluminumkilled steel. The refractory shape has a metal contacting surface whichis resistant to both steel erosion and the build-up of alumina. Moreparticularly, a layer is formed along a selected molten metal contactingsurface by first firing the pressed body in an oxidizing atmospherewherein the carbon is oxidized (decarburized) to form a porous, oxidizedzone on the selected steel contacting surfaces. The remaining surfaceportions of the body are glazed to protect against oxidation during thisfiring operation. A carbon free refractory slip or slurry is theninfiltrated into the porous oxidized zone to create an erosion andalumina build-up resistant surface layer therein. Benson theorized, andconfirmed in the laboratory, that carbon monoxide (CO) gas is generatedin conventional carbon containing refractories at steelmakingtemperatures and that the CO reacts with aluminum dissolved in themolten steel to form alumina at the refractory wall. While the methodand article taught in the Benson '370 patent are effective in preventingthe build-up of unwanted alumina, the procedure is somewhat timeconsuming and the resulting article is relatively expensive tomanufacture.

The present invention represents an improvement over the aforementionedmethods, compositions and materials and provides a refractorycomposition, method of manufacture and resultant refractory shape whichis extremely resistant to alumina build-up, thermal shock and erosion.The refractory composition of the invention is suitable for use inmaking integral nozzle liners or complete shapes for contact with liquidsteel and/or slag or where high hardness, wear and erosion resistanceare needed.

In the parent application, Ser. No. 08/490,792, certain compositions andmethods are disclosed which are suitable for making copressed orcomposite shapes, that is, shapes having a layer of the composition ofthe present invention copressed with a body having a conventionalrefractory composition. Upon firing, selected carbon free surfaces areformed on an underlying body of conventional refractory material. Thematerial of the instant invention is, by contrast, suitable for makingthe entire shape therefrom such as, for example, an entire pouringnozzle. Other shapes such as slide gate plate inserts, tundish nozzlesor inner nozzles, tundish nozzle plates, stopper rod heads, immersionthermocouple sheaths or slagline sleeves, for example, may be madewholly from or partially from the composition of the present inventionapplied to the metal contacting surfaces thereof to provide extraprotection in those areas against steel erosion, wear and/or aluminabuild-up. This is especially true in the case of aggressive steelshaving a high oxygen content which are not always killed (at all orcompletely) and cause severe erosion problems in conventionalrefractories.

In addition, a presently preferred composition of the present inventionis stronger than conventional alumina-graphite body materials permittingthe use of thinner cross sections so as to provide a significant savingsin material usage when the entire shape, such as a pouring tube, is madetherefrom. Such thinner cross sections also translate into lower partweights to provide easier handling and lower net shipping costs than thesame shapes made from a conventional alumina-graphite composition.

The present invention further provides a dense surface after firingwhich eliminates the need and expense of glazing which is conventionallyused in carbon bonded refractories to protect the underlying carbon fromoxidation during service. Still further, the method according to theinstant invention provides an efficient and cost effective process forforming these improved refractory shapes.

SUMMARY OF THE INVENTION

Briefly stated, the refractory composition according to the presentinvention consists essentially of in weight % on a dry mix basis, atleast about 70% of a refractory oxide, such as alumina; less than about10% total carbon; an effective amount of permeability reducers, plussecondary oxide grains in an amount up to about 25%. The material ispressed in the form of a desired shape and then subsequently heated inan oxidizing atmosphere at a temperature of about 900°-1400° C. forabout 5-20 minutes, depending upon the temperature. During this heatingstep, the carbon present in the surface of the shape (exposed to theatmosphere) is oxidized to produce a carbon free, porous layer therein,typically between about 2-5 mm in depth. Simultaneously, during theheating step, as the decarburization of the surface proceeds, asintering/densification mechanism also actively progresses. Themechanism causes densification of the refractory grains in the carbonfree surface layer resulting in a dense, oxide bond having very lowporosity and, hence, low gas permeability. The densification occurringwithin the oxidized carbon free surface layer continues rapidly to apoint where infiltration of ambient oxygen into the refractory shape canno longer occur. Thus, at a depth beyond the densified, oxide bonded,carbon free layer (on the order of about 2-5 mm), the underlyingrefractory material develops a carbon bonded structure having acomposition substantially the same as the original mix composition priorto the high temperature heating. This underlying carbon bonded structurein the body of the refractory shape is desirable, particularly withrespect to the enhanced thermal shock resistance resulting therefrom.The heating step is preferably accomplished during a conventionalpreheating operation conducted prior to use. A conventional oxy-gasfired flame sometimes used in such preheating operations produces atemperature of about 1200° C. and is preferably conducted for a periodof about 30 to 45 minutes.

The composition of the present invention may be used to form a selectedmolten metal contacting surface and/or to form a hard, wear resistantsurface on any commonly used refractory shape wherein the balance of theshaped body is made from a copressed conventional carbon bonded materialsuch as, for example, alumina-graphite. More preferably, however, anentire pressed refractory shape is made from the material of the presentinvention. A preferred composition for making an entire shape, accordingto the present invention, contains less than 9% total carbon, of whichapproximately 5% by weight is graphite with the balance of the carbonbeing that contained in the binder system. Approximately 1/4 of thetotal carbon content is volatized from the binder system when thepressed shape is cured at low temperature prior to firing. Typicalpreferred carbon contents in the underlying body of the fired shape mayrange between 2%-15% and, more preferably, between about 3.5% to about7% by weight. The composition also more preferably contains about 2%-8%by weight of a metallic phase which oxidizes during firing to form areaction phase closing off the pores to prevent further migration ofoxygen into the material which thus limits the depth of oxidation duringthermal treatment. In all instances, the outer surface of the materialis characterized by the presence of the dense, carbon free layer afterfiring in air or in an otherwise oxidizing atmosphere. The dense, carbonfree surface layer is characterized by having high hardness and wearresistance. The surface is also erosion resistant and resists build-upof alumina due to the absence of carbon in the surface of therefractory. The dense surface also prevents migration of any gas speciestherethrough, such as CO, which further prevents the occurrence ofalumina build-up problems. These features as well as other aspects andadvantages of the invention will become evident when reference is madeto the following detailed description taken with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional, side elevation view of a submerged entrynozzle having a dense, carbon free liner of the present invention formedtherein;

FIG. 2 is a slightly enlarged, cross sectional view of the sidewall,taken at area II--II of FIG. 1;

FIG. 3 is a cross sectional, side elevation view of a submerged entrynozzle similar to FIG. 1 wherein the entire shape is made from thematerial of the present invention;

FIG. 4 is a slightly enlarged, cross sectional view of the sidewall ofthe nozzle shown in FIG. 3, taken at area IV--IV thereof;

FIG. 5 is a cross sectional, side elevation view of a lower plate andintegral pouring nozzle for use on a tundish sliding gate valve havingselected portions made from the composition and method of the presentinvention;

FIG. 6 is a cross sectional, side elevation view of a stopper rod foruse in a tundish having a nose portion made from the material of theinvention;

FIG. 7 is a cross sectional, side elevation view of a tundish nozzle orinner nozzle for a tundish made according to the present invention; and

FIG. 8 is a cross sectional, side elevation view of a tundish nozzleplate made according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings, one presently preferred embodiment of asubmerged entry nozzle 2 according to the present invention is shown inFIG. 1. Pouring tubes or nozzles of the general configuration depictedin FIG. 1 are commonly used in the continuous casting of steel. Thenozzle 2 includes a body 4 made from an isostatically pressed and firedrefractory material, such as a conventional, carbon bondedalumina-graphite. A known alumina graphite composition for body 4 mayinclude the following constituents, in % by weight, about: 32% carbon;52% alumina; 14% silica; and 2% other additions. The carbon is derivedprincipally from graphite plus a lesser amount from the carbonaceousbinder such as, for example, a known pitch or a resin binder system.

The nozzle 2 has a teeming orifice or bore 6 extending axiallytherethrough. The axial bore 6 extends from a top flanged end 8 andterminates at a floor or blind end 10 near the bottom end 12 of thenozzle. A plurality of openings forming casting orifices 14 extendoutwardly through the body 4 communicating with the bore 6 adjacent thefloor 10 to permit the delivery of molten steel to a continuous castingmold (not shown). A slagline sleeve 14 of an erosion and corrosionresistant material such as, for example, zirconia-graphite, isisostatically copressed with the body 4. The nozzle 2 thus far describedis conventional and well-known in the art. In conventional practice,after pressing, a carbon bonded refractory shape is fired at atemperature of about 1000° C. for a period of one hour or more in areducing or otherwise non-oxidizing atmosphere to protect the carbonphase. The external surface of the body is also coated with aconventional glaze 18 to form a glassy layer thereon to further protectthe carbon from oxidation during preheating and in service. The glaze 18may be applied in a thickness of up to about 5 mm with a thickness ofabout 3 mm being presently preferred.

The nozzle 2 depicted in FIG. 1 has a selected molten metal contactingsurface portion forming a liner portion 20 which coincides with the bore6 as well as the casting orifices 14 and floor 10 made from thecomposition according to the present invention. As seen in FIG. 1, theliner portion 20 forms a jacket or barrier along the surfaces of thenozzle bore 6 in contact with the molten steel where deposition ofalumina and blockage are likely to occur. The liner portion 20 in onepresently preferred formulation is principally made up of aluminamaterial, on the order of at least 60 weight % or more. The particlesize distribution and type of alumina used in the refractory mix isimportant because it influences the sintering/densification rate as wellas the final density levels achieved in the fired material. The aluminaportion of the composition of liner portion 20 preferably contains about50-75% calcined alumina. About 25-40% by weight of the alumina mix ismade up of relatively coarse grains having a U.S. mesh screen sizegreater than 65. About 10-30% by weight of the alumina is finer than a-325 mesh. Such a mixture of aluminas preferably comprise at least about70% by weight of the composition of the material mix. The balance of thecomposition includes less than about 10% total carbon comprising about5% graphite and about 5% binder carbon. A secondary refractory grainsuch as, for example, zirconia mullite, is also preferably present in anamount up to about 20% by weight. In addition, the compositionpreferably contains about 1-4% of very free reactive silica, such as"fumed" or micro silica to react with the alumina to produce a fractionof mullite. Densification of the liner 20 is further aided by theoccurrence of the mullite reaction which creates a ceramic bond betweenadjacent particles at relatively low temperatures-on the order of1000°-1200° C. Additional densification promoters or permeabilityreducers such as one or more of an oxidizable metal, borax, boroncarbide, boron nitride and/or other boride glass formers are added ineffective amounts (1-6 weight %) to further increase the densificationof the material during firing.

The refractory powder mixture making up the liner portion 20 ispreferably isostatically copressed in a mold along with the body 4 andslagline sleeve 16 to produce the nozzle 2 shown in FIG. 1. The nozzle2, after pressing, is heat treated to moderate temperatures (800°-1000°C.) to produce the carbon bond phase. The shape is then given a firingtreatment either by the manufacturer or conducted by the end user by wayof a preheating treatment prior to use in a continuous casting operationto develop the final characteristics desired in the invention. Aconventional gas preheat burner is employed having a flame temperatureof above 800° C. and preferably between 10000°-1500° C.

During firing, the surface of the liner 20 coinciding with the nozzlebore 6 is directly exposed to oxygen. The oxygen at this hightemperature immediately reacts with the carbon constituent in the liner20, causing a decarburization along the surface. By way of example, thesleeve 20 may have a thickness of about 10 mm while the decarburizedlayer formed during firing will preferably be about 3-5 mm in thickness.Two simultaneous phenomena occur during the firing step, namely,oxidation and densification. On one hand, oxidation of the carbon takesplace which increases the permeability/porosity of the liner 20 inproportion with the amount of carbon present. For this reason, thecarbon content of the liner is preferably held to no more than 10 weight%, and more preferably the carbon is limited to no more than 9 weight %,and still more preferably, to a level of no more than about 5 weight %.While the oxidation of the carbon proceeds, the densification of theceramic particles in the decarburized or carbon free outer region 20' ofliner 20 also takes place simultaneously therewith. Thus, the porous,decarburized outer surface begins to densify and the pores between therefractory grains disappear as densification progresses. Suchdensification eventually prevents the further infiltration ofatmospheric oxygen into the interior of the sleeve 20 to prohibitfurther oxidation/decarburization within the balance of the carboncontaining refractory material. For the purposes of the presentinvention, it is necessary that the densification of the surface layerproceed rapidly in order to outpace the oxidation rate. In other words,the densification should take place relatively quickly so as to preventcomplete decarburization by oxidation of the balance of the material.For this reason, the permeability reducers mentioned above are employedso as to accelerate the densification of the porous oxidized layer.

FIG. 2 depicts an enlarged cross section of the sidewall of nozzle 2showing the body 4 and the liner portion 20 after firing. It will beobserved that the liner 20, after the preheating firing step, includesan outer, carbon free, densified layer 20' and an inner sublayer 20".The inner sublayer 20" contains carbon due to the fact that it wasprotected from oxidation by the then densifying outer layer 20'. Asstated above, the densification of layer 20' prohibits migration ofoxygen to the sublayer 20". Hence, the composition of sublayer 20"remains identical to the original composition of the liner 20 prior tothe firing step. Layer 20", of course, is densified during thepreheating operation and during service and further bonds at itsinterface 21 with the refractory body 4 by virtue of the similar carbonbinder systems employed in the ceramic mixes forming the body 4 and theliner 20. The outer, carbon free, densified layer 20' of the liner 20 iscoextensive with the bore 6 and prevents migration of CO gas from thesublayer 20" or from body 4 into the molten steel, thus preventing theunwanted generation of alumina within the nozzle 2.

Ideally, the temperature of the nozzle 2 should be raised rapidly duringthe preheating step to a temperature of at least 1000° C. and, morepreferably, to at least about 1200° C. within a time of between about 20minutes to about 45 minutes. Heating rates of from about 25°/minute toabout 40°/minute up to at least 1200° C. are presently preferred heatingrates for use in preheating/firing the liner 20. This heating rate rangeresults in heating times of between about 30 minutes to 45 minutes toreach 1200° C. The nozzle 2 is then held at this 1200° C. temperature,or slightly higher, for about 30 minutes minimum to insure properdensification/sintering of the layers 20', 20" as well as the body 4.The preheated shape is then ready for service.

By way of example, in practice, a 615,000 BTU/hour oxy-gas burner flame(not shown) is directed vertically down the inlet at top 8 of the bore 6of the nozzle 2 for satisfactory preheating and firing of the liner 20and body 4. Firing may also be accomplished by the use of two oxy-gasburners (not shown) directed in the casting ports 14 at the bottom 12 ofthe nozzle 2. In the above example, the nozzle 2 of FIGS. 1 and 2 had awall thickness of 28 mm. The wall thickness of the refractory shape tobe heat treated will influence the temperature sensed on the outsidesurface thereof. Naturally, heavier walled shapes will exhibit lowersurface temperatures than thinner walled shapes, when fired internally,assuming the same refractory materials are used in each.

The layer of glaze material 18 also densifies along the outer surfacesof the body 2 during heating to prevent unwanted oxidation of the carbonin the conventional carbon bonded alumina, and zirconia-graphitematerials during preheating and use. It will be noted that the glaze,however, is not applied on the surface of the line 20 since it wouldprevent the desired oxidation of the layer 20' during the preheatingoperation. In use, it is necessary that the liner 20 present an outersurface layer 20' which is not only dense, but also carbon free. Thedense, low permeability, carbon free layer 20' prevents any inwardmigration of oxygen to react with carbon as well as the reversemigration of CO gas to the outer surface, thus preventing the occurrenceof unwanted alumina deposition problems heretofore prevalent in carboncontaining refractories.

A further and presently more preferred embodiment of the presentinvention is depicted in FIGS. 3 and 4 in which the entire continuouscasting nozzle 2', except for the slagline sleeve 16', is made from arefractory composition of the invention. Nozzle 2' is shaped like nozzle2 of FIG. 1 and also functions as a submerged pouring tube fortransferring molten steel from a tundish to a continuous casting mold(not shown). In the embodiment of FIGS. 3-4, a carbon free, oxidized anddensified layer 40 forms along all of the exposed surfaces of the nozzle2' after firing. As a result, no conventional glazes need be applied tothe shape since the presence of the carbon free, densified layer 40protects the carbon in the underlying body 4' from reaction with oxygen.In addition, the presence of the densified layer 40 prevents the reversemigration of carbon monoxide gas into the molten steel and prevents theformation of alumina therein, as previously discussed.

The sidewall of the pouring tube 2' made according to the embodimentshown in FIGS. 3 and 4 has a sidewall thickness of about one-half thatof a conventional nozzle made from an alumina-graphite material. Thisreduction in cross section results from the fact that the refractorycomposition of the present invention possesses superior strengthcompared with conventional alumina-graphite material. By way of example,a pouring tube or nozzle 2' made entirely (except for the slaglinesleeve 16') from a refractory mix of the present invention has a wallthickness of about 15 mm compared with a conventional alumina-graphitetube which typically has a sidewall thickness on the order of about28-30 mm. Naturally, this represents a significant material and weightsavings in a product which typically has a length on the order of 900 mm(about 3 feet). Due to the fact that the refractory composition of thepresent invention is stronger and more erosion resistant thanconventional alumina-graphite, the wall thickness may be reduced withoutsacrificing any mechanical strength or service life. In the sidewallshown in the slightly enlarged view of FIG. 4, the carbon free,densifled layer 40 on the inner and outer surfaces of the pouring tubeor nozzle 2' has a thickness of between about 2-4 mm, while the interiorbody portion 4' has a thickness on the order of about 10 mm. A presentlypreferred thickness for the layer 40 is about 2.5 mm. It will also benoted that the body portion 4' of the nozzle 2' is carbon bonded andcontains graphite so as to provide good thermal shock properties.

As is conventional in the manufacture of carbon bonded refractories, theisopressed nozzle is coked at a moderate temperature of between about800°-1000° C. for about 1-3 hours to establish the carbon bond phasethroughout the refractory body and the slagline sleeve. This cokingoperation is conducted after pressing or otherwise forming the shape andprior to the preheating/firing step. In order to protect the carbon(binder carbon and graphitic carbon) in a pressed refractory bodyagainst oxidation, conventional coking is either carried out in anon-oxidizing atmosphere or the pressed body is first coated with aglaze 18 (as shown in FIG. 1) if the shape is to be coked in an open,oxygen containing atmosphere. The glaze layer 18 provides a glassybarrier on the surface and prevents the atmospheric oxygen from reactingwith the carbon constituent in the body 4. This is conventionalpractice. In the present invention, one composition of the liner 20 issuch that the carbon in the liner portion (bond phase and graphite) mustbe protected against excessive oxidation during the coking step, usuallywith a protective atmosphere. The coked composition is then preheated tooxidize and densify the surface layer, as described above.

The instant invention also includes another presently preferredcomposition for the liner 20 or for the entire nozzle 2' which can besuccessfully coked in an oxygen containing atmosphere. This preferredcomposition develops a controlled rate of oxidation during coking in anopen atmosphere which proceeds to a depth of about 2-2.5 mm and thenstops. This advantageous feature is achieved by the addition of acontrolled amount of an oxidizable metal to the composition. Duringoxidation, as for example occurring in an open atmosphere coking step,the oxidizable metal constituent, such as silicon metal or aluminummetal, oxidizes, resulting in the formation of a glassy phase. Thisglassy phase then behaves as a passive oxidation layer in which furtheroxidation of the mix becomes dependent on the transport of gaseousoxygen (O₂) through the newly formed glassy phase to the unoxidizedcarbon and metal in the mix. The passive oxidation follows the rules ofparabolic kinetics, that is, the oxidation rate decreases dramaticallyas the thickness of the glassy phase increases. After coking, thispreferred metal containing mix provides a surface which has a porous,oxidized (carbon free) layer on its outer surface of controlled depth of2-2.5 mm in thickness. Upon pre-heating of the shape 2, the open poresin the previously formed oxidized layer close as densificationprogresses at preheating temperatures above 1000°-1200° C. to producethe desired dense, gas impermeable, carbon free layer 20' of FIG. 2 orlayer 40 of FIGS. 3 and 4.

One presently preferred formulation of a refractory material mix forisopressing a carbon free liner 20 or for forming a complete refractoryshape 2' which may be coked in an open atmosphere, as discussedimmediately above, consists essentially of the following constituents inweight %:

    ______________________________________                                                Alumina (Al.sub.2 O.sub.3)                                                                         50-70%                                                   Zirconia mullite (ZrO.sub.2.(3Al.sub.2 O.sub.3.2SiO.sub.2))                                         0-20%                                                   Silicon metal (Si)   2-8%                                             mix     Boron carbide (B.sub.4 C)                                                                          1-3%                                                     Micro silica (SiO.sub.2)                                                                           1-3%                                                     Carbon (C) (Graphite + binder)                                                                      3-13%                                           ______________________________________                                    

In this mixture, the alumina grains are sized as discussed above toprovide good packing density and densifying characteristics. One or bothof the permeability reducers, micro silica or boron carbide, should beused. The mixture is then placed in a mold and isostatically pressedinto a desired shape, such as the liner 20 of FIG. 1, or the completenozzle 2' of FIG. 3. After pressing, the shape is fired in an openatmosphere at about 800° C. to develop the carbon bond phase and to formthe oxidized, carbon free, porous surface layer. Due to the presence ofsilicon metal in the mix, the oxidized layer stops forming after aperiod of time at a depth of about 2-2.5 mm. The shape is then preheatedprior to service at a preferred temperature on the order of about 1200°C. to permit the porous, oxidized layer to densify to form the dense,carbon free layer 20' of FIG. 2 or layer 40 of FIGS. 3 and 4. The carbonfree, dense layer 20' or 40 develops an oxide bond between the adjacentceramic particles as densification progresses and the pores disappearduring the firing/preheating step. The underlying cured, coked andfired/preheated layer 20" of FIG. 2 and body 4' of FIGS. 3 and 4 exhibita carbon bonded alumina graphite composition similar to the original mixcomposition, except that a portion of the initial carbon content in thebinder has volatized out during thermal treatment to yield a reducedcarbon content in portions 20" and 4'. Due to the fact that theunderlying portion 20" and body 4' contain a predominately carbonbonded, alumina-graphite composition, thermal shock properties are notsacrificed.

A presently preferred, higher strength composition of the presentinvention for the manufacture of an entire refractory shape is set forthin Table 1. This is a composition of the pressed refractory shape, thatis after isostatic pressing and coking, but prior to preheating/firing.

                  TABLE 1                                                         ______________________________________                                        Composition                                                                   Constituent      Range (wt. %)                                                                            Nominal (wt. %)                                   ______________________________________                                        Alumina (Al.sub.2 O.sub.3)                                                                     60-85      75                                                Micro silica (SiO.sub.2)                                                                       1-3        2                                                 Zirconia mullite  3-10      6                                                 (ZrO.sub.2.(3Al.sub.2 O.sub.3.2SiO.sub.2))                                    Boron carbide (B.sub.4 C)                                                                      2-5        4                                                 Silicon metal (Si)                                                                             2-6        4                                                 Carbon (C) (Graphite + binder)                                                                  3-13      7                                                 Incidental constituents:                                                                       Balance    Balance                                           ______________________________________                                    

Table 2 demonstrates that the strength of the materials of the inventionis relatively high. The material of the present invention is compared inTable 2 with the oxidized and alumina infiltrated, alumina-graphitematerial disclosed in U.S. Pat. No. 5,370,370 which is considered to bevery strong but also very expensive to manufacture. The strength, asmeasured by the modulus of rupture (MOR) values at 1480° C., indicatesthe high strength of these materials when subjected to elevatedtemperatures for about 10 minutes. In a further test not listed in Table2, a sample piece made from the composition of the present invention wasleft in the furnace for several hours to completely oxidize and sinterthe material. This carbon free, completely oxidized and densified sampleof the present composition exhibited an MOR at 1480° C. of 1700 psiwhich is also considered good. Such high strength is important to theintegrity and erosion resistance of the material. Thermal expansioncharacteristics are also important, especially with respect to thermalshock and cracking caused by differential expansion/shrinkage and thethermal expansion properties of the materials of the invention areexcellent compared with conventional unoxidized materials.

                  TABLE 2                                                         ______________________________________                                        Physical Properties                                                           Property       Infiltrated*                                                                            Present Invention                                    ______________________________________                                        MOR (RT):      4000      4000-5000                                            MOR (1480° C.):                                                                       2000      1200-2000                                            App. Porosity: 20.0      15.5-17.0                                            App. Sp. Gravity:                                                                            ˜3.8                                                                              3.60-3.70                                            Thermal Expansion,                                                            RT - 1600° C.:                                                                        --        +1.4%                                                RT - 1600 - RT:                                                                              --        0.0 to -1.2%                                         Depth of C-free,                                                                             5-10       2-10                                                oxidized layer:                                                               ______________________________________                                         *U.S. Pat. No. 5,370,370  oxidized and infiltrated with Al.sub.2 O.sub.3 

Table 3 reports the depth of oxidation in millimeters for preheating orfiring at various times and temperatures for a material of the presentinvention having a nominal composition reported in Table 1. The materialwas formed into cubes measuring 25 mm per side. The cubes were oxidizedon one surface only in a resistance type furnace. Oxidation depthmeasurements reported in Table 3 indicate that the depth of theoxidized, carbon free layer can be controlled by selecting the heatingtemperature and the heating time. Table 3, likewise, demonstrates thatat the higher temperatures, 1300°-1400° C., the densification phenomenontakes precedent over the oxidizing phenomenon and quickly closes off thepores to prevent the formation of a carbon free layer of any significantdepth. Hence, a preheating/firing temperature of 1200° C., or slightlyin excess thereof, is preferred to provide a desired, carbon free,oxidized layer of about 2-5 mm in depth.

                  TABLE 3                                                         ______________________________________                                        Oxidation Depth                                                               Temp      1 hour        2 hours 3 hours                                       ______________________________________                                        1200° C.                                                                         3.0 mm        3.8 mm  5.0 mm                                        1300° C.                                                                         3.0 mm        3.3 mm  3.1 mm                                        1400° C.                                                                         0.7 mm        0.5 mm  0.4 mm                                        ______________________________________                                    

Actual pouring tubes or nozzles 2 were manufactured having liners 20made from material of the present invention. The test nozzles were usedin the actual casting of steel in an operating steel plant. Castingtrials indicate that the erosion resistance and the resistance toalumina build-up in the material of the invention were good.

The above-described pouring tubes or nozzles 2, 2' are also referred toin the art as submerged entry shrouds ("SES") or submerged entry nozzles("SEN"). In addition to these SES and SEN products, the instantinvention is useful in the manufacture of other metal handlingrefractory shapes. FIG. 5 depicts a so-called monoblock submergedpouring tube and integral slide plate 60 for use in a slide gate valvefor metering the flow of molten steel from a tundish to a continuouscasting mold. The shape 60 can be made entirely from the composition ofthe present invention or it can have selected portions made therefrom.For example, the top plate 62 and inner bore liner 64 are made from acomposition of the invention and thermally treated according to themethod of the present invention to form a carbon free, dense layer alongthese selected surfaces. As depicted in FIG. 5, after preheating/firing,a dense, carbon free layer 63 is formed along the upper surface of thetop plate 62 and a dense, carbon free layer 65 also forms around theouter surface of the bore liner 64. The wear resistant property of thehard, carbon free, dense layer 63 is advantageous to the operation of aslide gate plate portion 62, while the anti-alumina depositionproperties of the carbon free layer 65 along the liner 64 are alsobeneficial in extending the service life of the pouring nozzle. Thecomposite shape 60 also includes a body portion 66 made from aconventional refractory mix such as alumina-graphite with a conventionalslagline sleeve 68. All of these parts would be copressed andsubsequently thermally treated, as previously described, to produce thedesired carbon free, densified layer 63 along the top of the plateportion 62 and layer 65 on the surface of the bore liner 64. Aconventional steel jacket or can 61 may encase the top portion of themonoblock shape 60 to provide protection and added strength when thepiece is operable in the slide gate valve.

The material and method of the present invention are also useful inmanufacturing a tundish stopper rod 70 shown in FIG. 6. The stopper rodhas a cylindrical body 72 made from a conventional refractorycomposition such as, for example, alumina-graphite. The bullet-shapednose 74 of the stopper rod is made from the material of the presentinvention and is copressed with the body 72 in a known manner. The body72 preferably has a conventional glaze layer 73 applied around its outersurface to protect the carbon from oxidation. Upon preheating of thestopper rod 70 prior to service, using the previously described time andtemperature, a dense, carbon free layer 75 is formed along the outersurface of the nose 74. The erosion resistant properties of the dense,carbon free layer 75 are particularly well-suited for the useenvironment of a stopper rod.

The interface 76 between the body 72 and the nose 74 forms a strongjoint due to the copressing and due to the fact that similar oridentical carbonaceous binder systems are employed in the mixes makingup the body and the nose. In this manner, a carbon bond is establishedacross the interface 76 after thermal treatment. In addition, similarthermal expansion properties can be developed between the body and noseportions by employing a predominately alumina-graphite composition ineach.

Another steelmaking refractory shape which may be made from thecomposition, and according to the method of the present invention, is atundish nozzle or inner nozzle 80 shown in FIG. 7. The inner nozzle 80is fitted in a known manner in the bottom floor wall of a tundish topermit molten metal to flow from the tundish to a lower pouring tube andcontinuous casting mold (not shown). The nozzle 80 has an axial bore 82through which the molten metal passes a radially shaped portion 84 isformed at the top of the nozzle bore 82 and is adapted to provide a seatfor the nose portion of a stopper rod, such as the stopper rod nose 75of FIG. 6. Upon preheating, as previously described, the exposed surfaceof the nozzle 80 is decarburized and densified to form a carbon free,dense layer 86 therearound. The inner core or body 88 of the nozzle 80,as in the previously discussed embodiments, comprises a desired carbonbonded refractory oxide structure after firing/preheating. It will befurther appreciated that the carbon free, dense layer 86 formed alongthe radially shaped seating portion 84 provides a hard, erosionresistant surface to counter the effects of flowing molten steel. Thislayer 86 also presents a wear resistant seating surface for consistentmating with the stopper rod nose. In addition, the carbon free, denselayer 86 around the nozzle bore 82 also prevents unwanted aluminaformation and bore plugging problems previously discussed. It is alsoknown that erosion or alumina deposition in the stopper rod seatingportion 84 may result in an uneven surface therealong and can creategaps between it and the stopper rod nose when the stopper rod islowered, creating molten metal leakage. This harmful condition iseliminated by the instant invention.

Still yet another steelmaking refractory shape which may be made fromthe present composition, and according to the method of the invention,is a tundish nozzle plate 90 depicted in FIG. 8. The nozzle 90 is alsofitted in the bottom of a tundish similar to the nozzle 80 of FIG. 8 butis used in installations which employ a lower changeable pouring tube(not shown). In this regard, the nozzle 90 includes a lower flat plateportion 92 which is adapted in use to engage a flat upper plate carriedby the changeable pouring tube. In such installations, a truly flat,hard and wear resistant surface in the plate portion 92 is desirable.Such properties are obtained when the nozzle 90 is made from therefractory material of the present invention. After firing/preheating,the previously described carbon free, dense layer 94 is formed along theexposed surface of the shape to provide these desired properties. As inthe previously described nozzle 80, the nozzle 90 of FIG. 8 also has aradially shaped stopper rod nose engaging a seat portion 96 and a bore98 having the beneficial erosion resistant and anti-alumina depositioncharacteristics to greatly enhance the service life and overallperformance of the nozzle. The inner core or body 99 of the nozzle 90also possesses the desired carbon bonded structure which makes the shapewell-suited for high temperature service. As in the previously describedshapes, the nozzles 80 and 90 required no conventional glazing toprotect the graphite and carbon bond constituents of their respectiveinner cores or bodies 88 and 99 by virtue of the presence of the carbonfree, dense layers 86 and 94, respectively.

While the examples of the presently preferred compositions discussedherein are comprised predominately of alumina-graphite material, it willbe understood by persons skilled in the art that the major portion ofthe composition could be one or more other refractory oxide-graphitesystems such as, for example, zirconia-graphite, magnesia-graphite,spinel-graphite, mullite-graphite, silica-graphite and mixtures thereof.The critical feature in any refractory oxide system employed is to limitthe total carbon content to less than about 10% and to include thedensification promoters/permeability reducers in the mix. In order tolimit the depth of the oxide layer when firing in an oxygen bearingatmosphere, it is necessary to have an oxidizable metal, preferablysilicon, in the refractory mix. Other metals may be substituted forsilicon such as, for example, alloys of silicon and aluminum, molybdenumor cobalt may be used for this purpose. Other permeability reducers suchas borax, boron carbide, other borides and glass formers may also beused to close off the pores beneath the oxidized layer so as to protectthe underlying carbon in the body. The pressed shape is subjected to thepreviously discussed thermal treatment to first create the oxidized,carbon free layer and then this porous, carbon free layer is densifiedto develop the desired dense, gas impermeable layer on the outer surfaceof the shape. In all of these refractory oxide formulations, inwardmigration of oxygen and outward migration of carbon monoxide to and fromthe carbon containing body are prohibited by the dense carbon free layerto prevent alumina deposition/blockage problems.

In the case of a zirconia-graphite formulation made according to thepresent invention, the slagline sleeve 16' of FIG. 3, for example, maybe co-pressed with the body 4' having a composition containing aluminagraphite. In such a formulation, the outer surface layer 50 of theslagline sleeve 16' would consist of a carbon free, gas impermeablelayer of material similar to layer 40, thus eliminating the necessity ofusing any glazing materials since all exposed surfaces of the shape 2'are carbon free and dense.

It is also apparent that refractory shapes other than the abovedescribed pouring nozzles and stopper rod could be advantageouslymanufactured using the compositions of the present invention. Variousother shapes employed in the handling of molten steel or in othernon-metallurgical environments requiring high hardness, wear/erosionresistant surfaces, and/or thermal shock resistance are ideal candidatesfor the material and method of the present invention.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. The presentlypreferred embodiments described herein are meant to be illustrative onlyand not limiting as to the scope of the invention which is to be giventhe full breadth of the appended claims and any and all equivalentsthereof.

What is claimed is:
 1. A refractory shape having at least a selectedsurface with a dense, carbon free layer thereon, said surface made froma refractory mix containing one or more refractory oxides, less thanabout 10% by weight carbon, and an effective amount of permeabilityreducers to increase a rate of densification of said refractory oxidesduring heating, wherein, upon heating said refractory shape to aselected temperature, the carbon in said selected surface is oxidizedand the refractory oxides are densified to form said dense, carbon freelayer.
 2. The refractory shape of claim 1 comprising a body made from analumina-graphite containing material underlying said selected surfaceand wherein a major portion of said refractory oxides contained in saidselected surface comprises alumina.
 3. The refractory shape of claim 1wherein said shape is made entirely from the refractory composition ofsaid selected surface.
 4. The refractory shape of claim 1 wherein saidrefractory composition includes an oxidizable metal addition adapted tocontrol a depth of oxidation in said selected surface.
 5. The refractoryshape of claim 1 in the form of a nozzle for use in casting steel andwherein said selected surface is an area surrounding a bore of saidnozzle.
 6. A pressed refractory shape for use in handling molten steelmade entirely from a refractory mix containing one or more refractoryoxides selected from the group consisting of alumina, zirconia,magnesia, spinel, mullite and silica; less than about 10% by weightcarbon; from about 2% up to about 6% by weight of an oxidizable metal;and about 1% to 8% by weight of one or more permeability reducersselected from the group consisting of boron containing compounds andother glass formers and reactive silica, wherein, when said shape isheated to a temperature above about 1000° C., said refractory shape isadapted to form a dense, carbon free surface layer and having a carbonbonded structure underlying said surface layer.
 7. A refractory shapehaving a composition consisting essentially of in weight %:

    ______________________________________                                        Refractory oxide 65-75%                                                       Reactive silica  1-3%                                                         Boron compound   2-5%                                                         Oxidizable metal 2-5%                                                         Carbon            4-10%                                                       Incidental impurities                                                                          Balance,                                                     ______________________________________                                    

said shape being adapted, when heated to about 1200° C. in air, to forma carbon free, sintered layer along an exposed surface thereof forming abarrier to gas penetration.
 8. The refractory shape of claim 7 whereinsaid shape is a submerged pouring tube for use in the continuous castingof steel.
 9. The refractory shape of claim 7 wherein said shape is anose portion of a stopper rod for use in the continuous casting ofsteel.
 10. The refractory shape of claim 7 wherein said shape is aone-piece slide gate plate and pouring tube for use in continuouslycasting steel.
 11. The refractory shape of claim 7 wherein the saidshape is one of an inner nozzle and nozzle plate for a tundish for usein the continuous casting of steel.
 12. A method for making a refractoryshape having a dense, carbon free layer thereon, comprising the stepsof:a) providing a refractory mix containing more than about 65% byweight of one or more refractory oxides, less than about 10% by weightcarbon and an effective amount of permeability reducers to increase arate of densification of said refractory oxides during a heating step;b) pressing the mix to form a pressed shape having at least a selectedsurface formed of said refractory mix; and c) heating said pressed shapeto oxidize said selected surface to render said surface porous andcarbon free and further heating to densify said porous, carbon freesurface, to finally render said surface dense and impervious to gases.13. The method of claim 12 including coking said pressed shape at atemperature of about 800°-1000° C. to form a carbon bond phase in saidrefractory shape.
 14. The method of claim 13 wherein said coking stepcauses said surface to oxidize and wherein said further heating at atemperature of about 1200° C. causes said oxidized, porous surface todensify.
 15. A method of making a refractory shape for use in handlingmolten steel comprising the steps of:a) providing a refractory mixconsisting essentially of in weight %:

    ______________________________________                                        Refractory oxide 65-75%                                                       Reactive silica  1-3%                                                         Boron compound   2-5%                                                         Oxidizable metal 2-5%                                                         Carbon            4-10%                                                       Incidental impurities                                                                          Balance,                                                     ______________________________________                                    

b) forming the mix to produce a shape; c) coking the shape at atemperature of about 800°-1000° C. to form a carbon bonded structure inthe shape; d) heating the coked shape at a temperature of about1000°-1400° C. for oxidizing the carbon along exposed surfaces of saidshape and forming a porous, carbon free layer and for then densifyingthe porous, carbon free layer to form a dense, carbon free layer on theexposed surface overlying said carbon bonded structure.
 16. The methodof claim 15 wherein the shape is a submerged pouring tube for use incontinuous casting.
 17. The method of claim 15 wherein the shape is anose portion of a stopper rod.
 18. The method of claim 15 wherein saidshape is a one-piece slide gate plate and pouring tube for use incontinuously casting steel.
 19. The method of claim 15 wherein the shapeis one selected from the group consisting of an inner nozzle for atundish and a nozzle plate for a tundish.
 20. The method of claim 19wherein the heating step is conducted at a temperature of about 1200° C.21. The refractory shape of claim 1 in the form of a submerged pouringtube for use in casting steel wherein said selected surface includes allsurfaces of said nozzle including surfaces of a slagline sleevesurrounding an outer peripheral area of said pouring tube.
 22. Thesubmerged pouring tube of claim 21 wherein a body portion of said nozzleis made from one or more refractory oxides comprising predominantlyalumina and wherein the carbon is substantially graphite and whereinsaid slagline sleeve portion is made from refractory oxide comprisingpredominantly zirconia.
 23. The pressed refractory shape of claim 6 inthe form of a pouring tube for use in the continuous casting of steel,said tube comprising an elongated body portion having an axial boretherethrough and an integral slagline sleeve portion surrounding anouter peripheral area of said body, said body portion including aluminaas a predominate refractory oxide and said slagline sleeve portionincluding zirconia as a predominant refractory oxide in said slaglinesleeve.
 24. The submerged pouring tube of claim 8 including a slaglinesleeve surrounding an outer periphery of said tube, said slagline sleevehaving a composition including zirconia as a predominate refractoryoxide and wherein the carbon is substantially graphite, said slaglinesleeve also being adapted to form a carbon free, densifled outer layerwhen heated to about 1200° C. in air.
 25. A submerged pouring tube foruse in the continuous casting of steel, said tube comprising a pressedrefractory body having an axial bore for the passage of liquid steeltherethrough, said pouring tube also including a refractory slaglinesleeve portion copressed with the body and surrounding an outerperiphery of said body, said body and slagline sleeve being co-pressedfrom respective refractory mixes comprising in weight %:

    ______________________________________                                        refractory oxide                                                                              65-75%                                                        reactive silica 1-3%                                                          boron compound  2-5%                                                          oxidizable metal                                                                              2-5%                                                          carbon/graphite   4-10%,                                                      ______________________________________                                    

and wherein the refractory oxide in the respective refractory mix forsaid body is alumina and the refractory oxide in the respectiverefractory mix for said slagline sleeve is zirconia and wherein saidpouring tube, when preheated to about 1200° C. in air prior to service,is adapted to form a carbon free, densifled layer along surfaces of saidbody, bore and slagline sleeve thereof forming a barrier to gaspenetration.